The problematic of this project was to characterize and tune the mechanical properties of semicrystalline polymers, with the ultimate goal of mimicking human skin. This biomaterial has the characteristic of adapting its rigidity exponentially under stretching. To do so, we used a triblock elastomer (sPP-EPR-sPP). This semicrystalline polymer has a specific microstructure, which consists of stiff regions (crystallites) and soft regions (amorphous matrix, see figure 1).

Figure 1-Left: Sketch of the microstructure of a semicrystalline polymer. Stiff crystallites are embedded in a soft and stretchy amorphous matrix. Right: During uniaxial tensile tests, the crystallites rotate and align with the imposed axis, then deform plastically and form stiff fibrils. Source: Kramer2010.

We characterize the elastomer using a visco-elasto-plastic model. Monotonic tests (elasticity), as well as cycles at increasing strain (plasticity) and relaxation tests (viscosity modeled by Eyring law), were performed in order to extract each specific component of the model. The fine analysis of plasticity shows break in slope and allows definition of a unique point for this material, that is interpreted as the onset of fibrillation. This non linear elastomer (see figure 2) has been further characterized by the empirical Mooney-Rivlin model. This allows separate measurements of the stress induced by the crystal skeleton and the amorphous network. The elastomer was further tuned by gelification in mineral oil and drying in hexane. The procedure aims at reducing the amorphous network entanglement and thus increase toughness and stretchability (see figure 2) to reach mechanical properties quantitatively closer to that of human skin.

Figure 2-Top: Drying process of the neat block polymer. Homogeneous gelation is induced by use of mineral oil with 0.2 wt% BHT (antioxidant) at 190°C under vacuum during 90 minutes. The mineral oil is then extracted with hexane. Bottom: The whole procedure reduces entanglement in the rubbery phase, while increasing crystallinity and viscosity. This enhances the hardening and the toughness of the final sample.